1. Field of the Invention
The present invention relates to an identification information reader (interrogator), which performs short-range radio communication with a radio frequency identification (RFID) tag, and also relates to a printer including the identification information reader.
2. Description of the Related Art
“RFID” is a generic term for technologies that use radio waves, i.e., electromagnetic waves, to automatically identify objects. There are several methods of identification, but most of common is to store at least identification information (a serial number or the like) in an RFID tag affixed to an object, and read the identification information from the RFID tag. The RFID tag is an IC tag in which a microchip is associated with an antenna connected thereto to transmit the identification information to, for example, an RFID reader, as a reply to an interrogation from the RFID reader. If the RFID tag is of a passive type which does not have a power supply, it is powered by a radio wave that is, for example, a non-modulated carrier wave transmitted from the RFID reader. The RFID tag performs backscatter modulation, superimposing identification information on the radio wave, and transmits the modulated wave to the RFID reader. If the object approaching the RFID reader is not made of metal, the RFID reader is able to succeed radio communication with the RFID tag buried in the object. Therefore, the RFID tag has been utilized in various fields: distribution management to track down an article distributed through a distribution route and then adjust the production and inventory in accordance with the sales condition; history management to manage histories, such as the details of work performed through the process of distributing the article; and article management to check the whereabouts of an article, e.g., a book on a shelf of a library, which moves due to lending and returning.
Conventionally, a label printer is known, which uses a roll of label tape having a series of label sheets that respectively cover RFID tags arranged in series and are pasted on base paper, each label sheet being detachable from the base paper along with the corresponding RFID tag (for example, see Jpn. Pat. Appln. KOKAI Publication No. 2005-332318). The label printer feeds the label tape in one direction. During the feeding, the label printer performs an operation of printing characters or graphic symbols on the label sheets, and an operation of writing and reading identification data in and from the RFID tag.
The label printer disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2005-332318 includes an RFID reader writer and print mechanism. The RFID reader writer writes identification information in each RFID tag, and reads the written information therefrom. The print mechanism prints characters or graphic symbols on the label sheet corresponding to the RFID tag. The RFID reader writer has a radio-frequency (RF) transmission line for use in both transmission of a radio wave to the RFID tags and reception of a radio wave that has been modulated with at least the identification information and transmitted from the RFID tag. The RFID reader writer also has a transmitter that outputs a radio wave to be transmitted from an antenna, and a receiver that demodulates a radio wave received by the antenna to obtain the identification information. The transmitter is capable of modulating the radio wave with the identification information and outputting the modulated radio wave for transmission from the antenna, as writing of the identification information. When backscatter modulation is carried out in the RFID tag, the single antenna is used to both transmit and receive radio waves of the same frequency, as described above. For this purpose, the RFID reader writer has a directional coupler which guides the radio wave output from the transmitter to the antenna, and guides the radio wave received by the antenna to the receiver. The radio wave guided from the transmitter to the antenna and the radio wave guided from the antenna to the receiver are isolated by the directional coupler. For example, if a circulator is used as the directional coupler, the isolation is set to about 15 to 26 dB, thus preventing a leak of transmission power from the transmitter to the receiver.
The antenna is located on the base paper side opposite to the label sheet side, and adjacent to the print head of the print mechanism and the RFID tag at a distance of several millimeters. Therefore, the voltage standing wave ratio (VSWR) of the antenna increases due to an influence of the print head and the RFID tag, and reduces the isolation in the directional coupler. In other words, due to the increase in VSWR, that is, a decrease in return loss, the transmission power of the radio wave guided from the transmitter to the antenna is significantly reflected by the antenna. In this case, part of the transmission power may be input to the receiver at a high level. If saturation occurs in the receiver by the transmission power, it will be difficult to obtain identification information in demodulation performed by the receiver.
This problem may be solved by antenna matching, which equalizes the antenna impedance of the RFID reader writer with respect to the RFID tag and the characteristic impedance of the radio-frequency transmission line, thereby reducing the VSWR. However, since the antenna shape of the RFID tag and the dielectric constant of the base paper are not standardized, it is not practical to adjust the antenna matching in conformity with them.